I like my Surefire G2 - have owned both the Xenon and LED versions. I found boith at Gander Mountain in the clearance bin for $20. I keep the LED version in my car first aid kit next to a Maglite Mini Solitaire and a few spare in-pack batteries.

G2 only requires a decent set of rings to mount onto any rail. Weaver rings - $7, Walmart.

Ultrafire wf-502b. It throws its 900 lumens pretty well and can be had on Amazon for $10. I haven't actually used mine mounted on an airsoft or real steel gun yet, but it seems solid and reviews are good.